Mining operations have played an integral part in the development of the South African economic and political landscape from as early as 1870. Gold and coal mining have brought certain economic benefits thereby forming the backbone of the South African economy, South Africa being one of the main international suppliers of coal. However, these economic developments have come at significant environmental cost. Furthermore, water contamination which results from mining activity may pose a threat to human health. One of the main problems associated with coal and gold mines in South Africa is the generation of drainages with high concentrations of anions, most significantly sulfur dioxide anions (SO42−), and metals (mainly iron and manganese) which result from the oxidation of iron pyrite (FeS2), which is associated with the coal deposits in the Karoo super group and the gold deposits of the Witwatersrand Basin, to name but a few examples.
Although the coal mining super group of the Karoo and the gold deposits in the Witwatersrand Basin are given as specific examples, mine drainage or, more precisely, acid mine drainage (AMD) or acid rock drainage, collectively called acid drainage (AD) is found around the world both as a result of naturally occurring processes and activities associated with land disturbances, such as highway construction and mining where acid-forming minerals are exposed at the surface of the earth. In fact Roman mines in Britain and Bronze Age workings in Spain still produce AMD and coal mines have been found to continue to contaminate rivers well after their closure. Upon exposure to oxidizing conditions, sulfide minerals are oxidized in the presence of water and oxygen to form highly acidic, sulfate-rich drainage.
Mining increases the exposed surface area of sulfur-bearing rocks allowing for excess acid generation beyond natural buffering capabilities found in host rock and water resources. Since large masses of sulfide minerals are exposed quickly during the mining and milling processes, the surrounding environment can often not attenuate the resulting low pH conditions. Furthermore, metals that were once part of the host rock become solubilized and exacerbate the effect of the highly acidic, sulfate-rich drainage.
As mentioned above, acid mine drainage (AMD) is produced by the oxidation of sulfide minerals, chiefly iron pyrite (FeS2). The reaction of pyrite with oxygen and water produces a solution of ferrous sulfate and sulfuric acid. Ferrous iron can further be oxidized producing additional acidity. The chemical reactions that take place as part of this process are detailed hereunder:
Oxidation of Pyrite by Oxygen in the Presence of Water:FeS2(s)+3.5O2(g)+2H2O(l)→Fe2+(aq)+2SO42−(aq)+2H+(aq)  (1)Oxidation of Pyrite by Fe2+ (Ferrous Ion)FeS2(s)+14Fe3++8H2O(l)→15Fe2+(aq)+2SO42−(aq)+16H+(aq)  (2)Oxidation of Fe2+ by Oxygen
Further oxidation of Fe2+ (ferrous iron) to Fe3+ (ferric iron) occurs when sufficient oxygen is dissolved in the water or when water is exposed to sufficient atmospheric oxygen (equation 3). This reaction is accelerated by the presence of oxidizing bacteria such as Acidithiobacillus ferrooxidans. Fe2++0.25O2+H+→Fe3++0.5H2O  (3)Precipitation of Fe3+ (Ferric Ion):
Ferric iron can either precipitate as Fe(OH)3, a red-orange precipitate (equation 4) seen in waters affected by acid rock drainage, or it can react directly with pyrite to produce more ferrous iron and acidity as shown by equation 5.2Fe3++6H2O→2Fe(OH)3(s)+6H+  (4)14Fe3++FeS2(s)+8H2O→2SO42−+15Fe2++16H+  (5)
When ferrous iron is produced (equation 5) and sufficient dissolved oxygen is present, the cycle of reactions 3 and 4 is perpetuated. Without dissolved oxygen, equation 5 will continue to completion and water will show elevated levels of ferrous iron. The rate of the overall acidification process is determined by equation 3. However, the rate of the overall acidification process can be increased by up to six fold when reactions 1 and 3 are catalyzed by bacteria, as briefly alluded to above in relation to reaction 3.
Hydrolysis reactions of many common metals also form precipitates and in doing so generate H+. These reactions commonly occur where mixing of acidic waters with substantial dissolved metals results in the precipitation of metal hydroxides.Al3++3H2O→Al(OH)3(s)+3H+  (6)
Other metals may be combined with sulfide in the form of, inter alia, chalcopyrite (CuFeS2), sphalerite ((Zn,Fe)S), covellite (CuS), and arsenopyrite (FeAsS) whereby oxidation and hydrolysis of these metal sulfide minerals release metals such as copper, zinc, iron, arsenic, nickel and lead into solution concomitant with acidity and sulfate production.
Although the chemistry of AMD generation is relatively straightforward, the final product is a function of the geology of the mining region, presence of micro-organisms, temperature and also of the availability of water and oxygen. These factors are highly variable from one region to another, and, for this reason, the prediction, prevention, containment and treatment of AMD must be considered carefully and with great specificity.
Moreover, the circum-neutral or alkaline pH of the alkaline mine drainage can also be produced due to: (i) a low content of sulphide minerals; (ii) the presence of monosulphides rather than pyrite or marcasite; (iii) a large pyrite grain-size limiting oxidation rate; (iv) neutralization of acid by carbonate or basic silicate minerals; (v) engineering factors (introduction of lime dust for explosion prevention; cement or rock flour during construction works); (vi) neutralization of acid by naturally highly alkaline groundwaters; (vii) circulating water not coming into effective contact with sulphide minerals; and (viii) oxygen not coming into direct contact with sulphide minerals or influent water being highly reducing.
The goal in treating AMD is ultimately to an increase in pH with the resultant neutralization and precipitation of heavy metals. A variety of treatment approaches ranging from physical, chemical and biological methods have been employed.
Physical treatment processes can be used for the treatment of MD. Such physical treatment processes are generally membrane-based methods such as ultra-filtration, electrodialysis and reverse osmosis, which result in a highly concentrated brine stream. However, the field of this particular invention relates directly to chemical and biological treatment processes or a combination of these methods and hence no further mention of physical treatment processes is made herein.
Mine drainage is a major cause of ground and surface water pollution in South Africa. Because such pollution can persist for decades and even centuries after the cessation of industrial activity, there is a pressing need to develop cheap, sustainable remedial methods.
There are currently two types of chemical and biological process for the remediation of MD, namely active and passive processes. Active processes involve the continuous application of alkaline materials to neutralize acidic mine waters and precipitate metals whilst passive processes utilize natural and constructed wetland systems. Each of these processes is dealt with in further detail hereunder:
Active Chemical Treatment
This process involves the addition of a chemical neutralizing agent to AMD effluent to be treated. This agent is alkaline (typically lime) and it raises the pH of the AMD which accelerates the oxidation of the ferrous iron (for which active aeration or the addition of a chemical oxidizing agent such as hydrogen peroxide is necessary) and causes many of the metals present in solution to precipitate as hydroxide and carbonates. This addition of alkaline material also produces an environment which is unfavorable to pyrite oxidation given that iron oxidizing bacteria require an acidic environment to promote optimum activity. This active chemical treatment process results in a large amount of iron rich sludge that may also contain other metals depending on the chemistry of the mine water treated. Despite its effectiveness this method is disadvantageous in that it results in the production of the iron rich sludge which must then be disposed of. Furthermore, this method tends to have high operational costs and requires constant monitoring.
Active Biological Treatment
In this system a sulfidogenic bioreactor is used to bioremediate AMD. In a sulfidogenic bioreactor the biogenic production of hydrogen sulfide (H2S) by Sulfate Reducing Bacteria (SRBs) is used to generate alkalinity and to precipitate metals as insoluble sulfides. These engineered systems have more predictable performance than their passive system counterparts. Furthermore, these systems allow for the selective recovery of heavy metals from MD, allowing for metals such as copper and zinc to be reused. Finally, these systems also facilitate the significant lowering of sulfate concentrations within the MD. However, these systems also have a number of disadvantages. Firstly, these bioreactors have large construction and operational costs. Furthermore, the sulfate reducing bacteria (SRB) used within these reactors are sensitive to even moderately acidic conditions meaning that pretreatment to remove the acidity in the AMD is required. The activity of the SRBs is rate limiting and hence both pretreatment and the potentially slowed reaction rates lead to further increased costs.
Passive Processes
Passive treatment systems improve water quality using only naturally available energy sources, for example, gravity, microbial metabolic energy and photosynthesis. Once built, such systems require infrequent maintenance. Examples of passive treatment systems include: wetland treatment systems, anoxic limestone drains and reducing or alkalinity producing systems. In many instances, these systems rely on bioremediation which is the transformation of a contaminant using biological agents to convert the material to less toxic forms. Wetland systems, which also have a biological component, have longer response times in regard to changes to the system, for example changes in the composition of influent water, making the system a much slower system overall when in operation. Further disadvantageously, these systems can show problems with clogging and loss of reactivity when exposed to MD with high concentrations of metals.
To address some of the above stated problems dispersed alkaline substrate (DAS) are used. The substrate comprise a fine-grained alkaline reagent (limestone, sand or magnesium oxide powder) mixed with a coarse inert matrix, such as wood chips, to increase reactivity and reduce passivation. These materials also provide high porosity and reduce the clogging problems. Typically calcium carbonate (CaCO3) dispersed alkaline substrate (DAS), hereinafter referred to as CaCO3-DAS or magnesium oxide (MgO) dispersed alkaline substrate, hereinafter referred to as MgO-DAS is used for this purpose. However, it has been found that the introduction of CaCO3-DAS or MgO-DAS does not remove sulfate (SO42−) and the iron (Fe) removal is also not complete, which is disadvantageous.
Finding a solution to remediate MD is not only a matter of environmental importance, but also one of protecting vulnerable, local communities that depend upon finite natural resources adversely affected by MD.
In addition gold and silver mines produce MDs with high concentration of cyanide as the industry uses sodium and potassium cyanide to recover these metals (equation 7 and 8). The cyanide leaching process also referred to as cyanidation is an established technology used in the extraction of gold and other metals such as silver, copper and zinc from oxidized ores and it accounts for up to 90% of global production.4Au+8NaCN+O2+2H2O→4Na[Au(CN)2]+4NaOH  (7)Ag2S+4NaCN+H2O→2Na[Ag(CN)2]+NaSH+NaOH  (8)
However, excessive use of cyanide for the dissolution of gold is associated with environmental risk. Cyanide, especially when in its free form HCN or CN—, can be very toxic, due to its high metabolic inhibition potential. It may degrade into cyanate (OCN—) which is of generally lower toxicity but which could still be problematic in the environment. Cyanide poisoning can occur through consumption of contaminated surface water or concurrent exposure through inhalation or skin absorption. The impact of MD and cyanide-containing wastewater or tailings on terrestrial and aquatic ecosystems is potentially enormous due to the huge volumes involved. If not adequately remediated, they can leach and pollute the main watershed.
Bio-augmentation to the bioreactors was tested to remove cyanide and high sulfate and iron concentrations. It means that the bioreactor was augmented with sulfate reducing bacteria. After 120 hours of acclimation period, the values obtained corresponded to a 90.6% removal of cyanide; which mean that the cyanide concentration in the final effluent had decreased from 436 μg/L to 41 μg/L; below SANS recommended levels. As well as, 94% of iron removal and 100% sulfate removal.
However, a study more wide and detailed must be carried out to determine the involved biochemicals processes in the cyanide degradation.
On the other hand, the nitrate represents a sort of emergent contamination for the groundwater reservoir. The nitrification come from farming activities; fertilizers, septic systems, and manure storage or spreading operations, they are the main focus of pollution. However, recently have been studied the relation between the nitrification of aquifers and the mining activities. During the last 60 years, the ammonium nitrate (NH4NO3) has been widely used as explosive in open pit, underground mining and quarries, as well as civil works. The explosion of ammonium nitrate releases gases as H2O, N2 and CO2. N2 can be easily oxidized to nitrate (NO3−) in contact with the oxygen of the air and it can be released to superficial or ground water, contributing to the water nitrification.
This bioreactor showed to lab scale the ability to remove high nitrate concentrations (3000 mg/L) contained in MD. The presence of the anaerobic bacteria in the MD such as sulfate reducing bacteria, mainly, which can, in the absence of sulfates, remove nitrate. The nitrate is used as electron acceptor and reduced up to N2 volatile, since it had not evidence of the neoformed minerals phase which contained N. The percentage of nitrate removal during of 6 months of running of the lab bioreactor was a 100% of nitrate removal which demonstrated the efficiency of this bioreactor to promote the bacteria settlement.
There is thus a clear need in the art to arrive at a solution for successfully remediating environmental media contaminated by MD as well as MD contaminated environments without suffering from the shortcomings associated with the techniques of the prior art.